A. Holographic Stereograms
Holographic stereograms originated in the late 1960's, one of the earliest publications in the field being that of Dominic J. DeBitetto entitled "Holographic Panoramic Stereograms Synthesized from White Light Recordings," Applied Optics., vol. 8, No. 8, August 1969, pages 1740-1741.
As explained in the DeBitetto article, in accordance with the most basic embodiment of this technique, a series of two-dimensional photographs of an object are taken at different perspectives and then sequentially projected onto a diffusion screen using a coherent light source. Each image on the screen is holographically recorded on a portion of a photographic plate using a reference beam and a vertical slit aperture located just in front of the plate. The slit aperture is moved between exposures so that the finished plate is composed of a series of strip holograms, each strip containing interference information regarding one of the two-dimensional images and the location of the slit aperture when that two-dimensional image was recorded.
Illumination of the finished plate with laser light reconstructs all of the two-dimensional images. It also reconstructs the location of the slit for each of the images. When a viewer's eyes are placed in the vicinity of the reconstructed slits, each eye sees a different perspective view. The viewer's mind fuses these views together and interprets them as a three dimensional object, thus achieving three dimensionality from two two-dimensional images, as in a conventional stereogram.
General discussions of holographic stereograms and more advanced embodiments of this technique, including embodiments in which the reconstruction can be performed using incoherent light, can be found in G. Saxby, Practical Holography, Prentice Hall, New York, 1988, pages 55-59 and 246-259; F. Unterseher, J. Hansen, and B. Schlesinger, Holography Handbook--Making Holograms the Easy Way, Ross Books, Berkeley, Calif., 1987, pages 288-293; King et al., "A New Approach to Computer-Generated Holography," Applied Optics, Vol. 9, 1970, pages 471-475; U.S. Pat. No. 3,832,027; W. Molteni, Jr., "Black and White Holographic Stereograms," Proceedings of the International Symposium on Display Holography, Vol. 1, 1982, pages 15-21; S. Benton, "Photographic Holography," SPIE, Vol. 391 Optics in Entertainment, 1983, pages 2-9; W. Molteni, Jr., "Natural Color Holographic Stereograms By Superimposing Three Rainbow Holograms," SPIE, Vol. 462 Optics in Entertainment II, 1984, pages 14-18; W. Molteni, Jr., "Computer-Aided Drawing of Holographic Stereograms," Proceedings of the International Symposium of Display Holography, Vol. 2, 1985, pages 223-230; and S. Benton, "Display Holography--An SPIE Critical Review of Technology," SPIE, Vol. 532 Holography, 1985, pages 8-13.
With regard to certain of the embodiments of the present invention, Iovine, U.S. Pat. No. 4,964,684, discloses the use of a liquid crystal display matrix to form the vertical slit aperture employed in preparing the strip holograms of a holographic stereogram; Benton, U.S. Pat. No, 4,445,749, discloses a process for producing a holographic stereogram which is substantially achromatic; and W. Molteni, Jr., "Shear Lens Photography for Holographic Stereograms," SPIE, Vol. 1461 Practical Holography V, 1991, pages 132-139, discloses the use of shear lens photography to prepare the two-dimensional images from which a holographic stereogram is constructed. Each of these techniques can be used in connection with the practice of the present invention.
The use of predistorted two-dimensional images to form an alcove holographic stereogram is discussed in Benton, U.S. Pat. No. 4,834,476. In one embodiment, a series of undistorted perspective views are decomposed into columns and the columns are redistributed among the views to provide the desired predistortion. In another embodiment, anamorphic ray tracing is used to achieve the predistortion. Significantly, with regard to the present invention, predistortions suitable for use with an alcove stereogram are not suitable for use with a heads-up or heads-down display of the type disclosed herein.
B. Heads-Up and Heads-Down Displays
Heads-up and heads-down displays allow a user to simultaneously view two images, namely, a first (primary) image not provided by the heads-up or heads-down display and a second (secondary) image produced by the display.
For example, when used in an automobile, a heads-up display allows the user to view instrument information while simultaneously viewing the highway. Similarly, a heads-down display can be used to present additional information, such as, warning lights or turn signals, superimposed upon a conventional in-dash instrument panel. Along these same lines, in a game application, such as a pin-ball machine, a heads-down display allows the user to view information or a character related to the game and simultaneously to view the field of action.
In general terms, heads-up and heads-down displays provide two sets of information to a user without requiring the user to substantially redirect his or her eyes away from a primary viewing window such as the windshield or instrument panel of an aircraft, automobile, or other mechanized object.
Conventional optical elements have been used to project heads-up and heads-down images into the user's viewing area (line of sight). In some cases, holographic optical elements have been used in place of conventional lenses and mirrors. See, for example, Nanba et al., U.S. Pat. No. 4,832,427, and Suzuki et al., U.S. Pat. No. 4,932,731. Both continuously varying information (e.g., vehicle speed, targeting information, fuel status, etc.) and discretely varying information (e.g., turn signals, warning lights, and the like) have been presented to the user by means of these techniques.
Heads-up and heads-down images have also been formed by holographic techniques wherein the hologram both replaces some or all of conventional optical elements and contains image information. See, for example, Moss, U.S. Pat. Nos. 4,737,001, 4,790,613, 4,818,048, 4,807,951, and 4,830,442, and Smith et al., U.S. Pat. No. 5,011,244. The present invention is concerned with these types of displays hereinafter referred to as "image-containing holographic second image displays" or "IHDs".
In the past, IHDs have used holograms formed by conventional processes such that the finished holograms have contained both horizontal and vertical parallax information. That is, the holograms have been prepared by illuminating an object with laser light, providing a conventional reference beam, forming an interference pattern between the light reflected or transmitted by the object and the reference beam, and recording the interference pattern in a recording medium as a diffraction pattern. The images presented by the holograms have comprised either three dimensional solids or two dimensional objects suspended in space. Holograms of this type will be referred to hereinafter as "conventional holograms."
In a typical application, the hologram of an IHD is laminated into or onto a vehicle window. See, for example, Freeman et al., U.S. Pat. No. 4,998,784. Light from a light source located in, for example, the vehicle's dashboard is projected onto the hologram where it interacts with the hologram's diffraction pattern and produces the desired heads-up image. Corresponding geometries are used with heads-down displays.
The prior IHDs using conventional holograms have suffered from a number of problems. These problems have originated from the diffractive nature of the holographic process. Because a hologram diffracts incident light, its performance is wavelength dependent. The more wavelengths which are incident upon the hologram, i.e., the greater the bandwidth of the incoming light, the lower the resolution of the resulting image. This effect is known in the art as chromatic or color blur.
A basic approach to the color blur problem is to make the hologram a reflection hologram as opposed to a transmission hologram. Reflection holograms through the Bragg effect automatically generate an image composed of a select envelope of wavelengths from among the wavelengths produced by the light source. Although this approach helps with the color blur problem, it does not provide a complete solution.
In addition to the use of reflection holograms, other approaches to the color blur problem include (a) limiting the bandwidth of the incoming light and (b) confining the holographic image volume to the plane of the holographic recording medium.
The bandwidth of the incoming light can be limited by using a laser light source. Alternatively, a broadband light source can be used and then highly filtered to substantially reduce the bandwidth of the output.
Due to cost, the laser approach is impractical for large scale applications, such as, the automotive market. Also, lasers operate at specific wavelengths which may not meet the requirements of a particular application. Similarly, narrow bandwidth filters are generally expensive and even the narrowest filters commercially available are too broad to actually solve the color blur problem for practical systems. Also, such filters are inefficient and thus require strong light sources which are themselves expensive and lead to problems in the areas of power drain and heat generation.
Confining the holographic image volume to the plane of the holographic recording medium addresses the color blur problem because with such a geometry, the different wavelengths of a broadband light source do not have an opportunity to spread apart before they form the holographic image. Unfortunately, this solution to the color blur problem severely limits the usefulness of IHDs which employ conventional holograms. This is because heads-up and heads-down displays work best when the plane of the perceived image is substantially perpendicular to the user's line of sight. Windshields of vehicles are typically not perpendicular to the user's line of sight, and instrument panels are often not perpendicular. Yet, as discussed above, this is where holograms used to produce heads-up and heads-down displays in vehicles are normally mounted. Accordingly, confining the holographic image volume to the plane of the holographic recording medium is in general not a practical solution to the color blur problem for an IHD employing a conventional hologram.
Smith, U.S. Pat. No. 4,981,332, proposes a solution to the color blur problem in which two holograms are used--one to produce the image and the other to compensate for the bandwidth of the light source. This approach introduces its own problems in that compensating holograms are costly and difficult to make. Also, the presence of this component increases the complexity of the system, as well as limiting its use to those situations where the light source, the image hologram, and the compensating hologram can be located relative to the user and to each other to achieve the required spectral compensation of the image.